Detectors based on semiconductors, for example like detectors with a photodiode or phototransistor matrix, allow the direct conversion of the incident radiation into an electrical signal, and have applications in several fields, such as medical imaging, luggage inspection and nuclear probes, for example.
A conventional spectrometer, that is a detector coupled with a processing and digitization circuit which determines the energy spectrum of the radiation received by the detector, is shown in FIG. 1 under the general reference 10. The spectrometer 10, designed for applications requiring a high counting rate of the incident photons on the detector, measures the energy of the photons using a lag line.
This spectrometer 10 comprises a semiconductor detector 12, for example made from CdZnTe, CdTe:Cl, or CdTe:In, polarized under a “HT” DC voltage via a resistance 14.
The detector 12 is subjected to a photon radiation, for example of gamma photons or X-ray photons, which ionize the semiconductor material. The charges thereby generated migrate toward electrodes (not shown) of the detector 12 due to the existence of the HT bias voltage.
The charge current “i” thus generated is recovered on a collector electrode 16 by a processing and digitization circuit 18, which counts the incident photons according to their energy in order to obtain the energy spectrum of the radiation.
The circuit 16 comprises a low noise charge preamplifier 20 mounted as an integrated circuit, a lag line energy measurement circuit 22, and an analog-to-digital converter 24.
The preamplifier 20 comprises a high gain amplifier 26 mounted in negative feedback with a parallel mounting of a capacitance 28, having the value C1, and a resistance 30, having the value R1, and also a capacitance 32 connected between the collector electrode 16 and the input of the amplifier 26.
FIG. 2 shows two pulses I1 and I2 of the current i resulting from the interaction of two photons with the detector 12, and FIG. 3 shows the corresponding voltage Vout at the output of the preamplifier 20.
The energy measurement circuit 22 comprises a lag line 32, connected to the output of the preamplifier 20 and applying thereto a predefined lag Δt and a first gain 34, connected to the output of the line 32.
The energy measurement circuit 22 also comprises a subtractor 36, connected to the preamplifier 20 and to the first gain 34 and calculating the difference between their outputs. Finally, a second gain 38 is provided at the output of the subtractor.
FIG. 4 shows, as a function of time “t”, a voltage Vout(t) at the output of the preamplifier 20, the corresponding lag voltage at the output of the first gain 34 Vout(t−Δt), and the corresponding voltage E(t) at the output of the energy measurement circuit 22.
The energy measurement circuit 22 therefore generates voltage pulses corresponding to the charge pulses generated by the incident photon on the detector. It is demonstrated that the amplitude of a pulse at the output of the circuit 22 is proportional to the energy of the corresponding photon if and only if the lag Δt is greater than the rise time Tm of the voltage Vout at the output of the preamplifier 20. FIGS. 5 to 10 show, as a function of time, the shape of a pulse E(t) at the output of the circuit 22 for Δt=Tm (FIGS. 5 to 7) and for Δt>Tm (FIGS. 8 to 10).
The output voltage of the measurement circuit 22 is therefore converted to a digital voltage by the converter 24, that is quantified and sampled in time at a predefined sampling frequency fe.
The digital voltage is then delivered to a digital processing module (not shown) which detects the digital voltage peaks to determine the energy of the photons and thereby construct an energy spectrum of the incident radiation on the detector.
The abovementioned system permits a high counting rate (about a few megaphotons per second).
However, the charge sampling and generation in the detection are asynchronous events. Accordingly, some sampling times may be inappropriate because not corresponding to the voltage peaks at the output of the measurement circuit. This problem is particularly serious when the peaks of this voltage display very steep slopes. To reduce this drawback, it is possible to significantly increase the sampling frequency of the converter, but at the cost of very expensive equipment with high power consumption.